Vulnerable plaques are atherosclerotic plaques causing in most cases mild to moderate narrowing of the arteries, and are comprised of a soft lipid core, and a very thin fibrous cap separating between the inner core and the lumen. The term “vulnerable” refers to the high tendency of these soft plaques to burst or rupture. Several factors may influence a plaque's stability and its tendency to rupture, including mechanical injury, inflammation, and infection. Progressive thrombosis and vasospasm may follow plaque rupture.
Mechanical stress acting on the thin cap, combined with insufficient support from the inner soft core, play an important role in plaque disruption. Once the thin cap ruptures, the thrombogenic content of the inner core is exposed to the blood circulation, initiating a thrombotic formation cascade that may occlude the arterial flow.
Repeated rupture and healing of a vulnerable plaque is one of the mechanisms, perhaps the dominant one, which creates artery stenosis. Moreover, a rupture of a plaque may lead to a direct arterial occlusion prior to any significant narrowing of the blood vessel.
Vulnerable plaques can be found in many blood vessels, including, but not limited to, coronary arteries; carotid arteries and peripheral arteries.
Most of the vulnerable plaques do not bulge inward. Instead, as a plaque grows, it often protrudes outward, into the wall of the artery, rather than into the channel-lumen where blood flows. In the following, vulnerable plaque is discussed as a plaque mass confined between a front surface, (also referred to as a cover or a cap), facing the inside of the blood vessel, and a back surface, forming part of the blood vessel and separating the blood vessel from its surroundings.
The term “Vulnerable Plaque” will refer in this document to any kind of rupture-prone plaque including, but not limited to coronary thin cap lipid reach plaques.
All types of atherosclerotic plaques with high likelihood of thrombotic complications and rapid progression are considered vulnerable plaques, as discussed in the article “From Vulnerable Plaque to Vulnerable Patient. A Call for New Definitions and Risk Assessment Strategies: Part I”, Naghavi M., et al, Circulation. 2003; 108:1664-1672, the contents of which is incorporated herein by reference.
U.S. Pat. No. 6,475,210 (hereinafter US '210), the disclosure of which is incorporated herein by reference, describes treating vulnerable plaque by applying to it energy, which may be ultrasound energy. This reference refers to the vulnerable plaque as being made of a proteinic cap covering a lipid pool, and teaches congealing the lipid pool. Such congealing apparently requires heating the lipids to at least 85° C., according to the article “Structural Basis for Thermal Stability of Human Low-Density Lipoprotein”, Jayaraman S., et al., Biochemistry 2005, 44, pp 3965-3971, the contents of which is incorporated herein by reference.
US patent application publication No. 2005-240249 (hereinafter US '249), the disclosure of which is incorporated herein by reference, seeks to provide heat to collagen of tissue to a temperature range of about 45° C. to 75° C. or higher for at least a few seconds to cause collagen to shrink a fraction of its original length.
US patent application publication No. 2003-0069525 (hereinafter US '525), the disclosure of which is incorporated herein by reference, teaches stabilizing atherosclerotic plaque by ultrasound heating to 47° C. or less, applied from outside the blood vessel.
An aneurysm is a localized, blood-filled dilation of a blood vessel. Aneurysms cavities are in many cases filled with blood thromboses (clots), generated due to relatively slow and circulatory blood flow inside the aneurysm cavity. Aneurysms are usually accompanied with thinning of the blood vessel wall.
Aneurysms most commonly occur in arteries at the base of the brain or in the aorta, The dilatation in a blood vessel can burst and lead to death at any time. The larger an aneurysm becomes, the more likely it is to burst. Current therapies include open surgery and endovascular stent-grafting. Some suggestions to treat aneurysm with heat were made, for instance, in U.S. Pat. Nos. 6,048,333; 6,375,668; U.S. Pat. No. 5,921,954; and PCT publication No. WO 99/53854.
In general, the absorption of ultrasonic energy in artery vessel wall is much bigger then in a blood thrombus (clot) and even bigger then in blood.